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Strict guidelines apply to drinking water. Nevertheless, germ contamination can occur. Coli bacteria and enterococci are detected particularly frequently.

Drinking water in Switzerland is one of the most strictly controlled foods. It consists of 99.9 percent water and is therefore one of the purest foods of all. Drinking water must be inconspicuous in terms of taste, smell and appearance. It must not endanger health with regard to the type and concentration of the microorganisms, parasites and contaminants it contains.

The Federal Food Safety and Veterinary Office (BLV) defines the legal provisions to ensure the high drinking water quality in Switzerland. The maximum values for the germs are specified in the Drinking Water Ordinance. Nevertheless, contaminated drinking water occurs again and again, as for example in August 2022 in Lucerne. E. coli bacteria and enterococci were also detected in several towns in canton Bern in the summer of 2022.

How is drinking water contaminated? What measures are taken to restore drinking water quality and who is responsible for quality control?

What germs are found in drinking water?

First of all: Drinking water is never completely free of germs. Even after proper treatment by the water supplier, it still contains microorganisms. These are either harmless water dwellers, bacteria or viruses that are present in such low concentrations that they pose no health risk. There are strict microbiological requirements and controls to ensure that the germ load is as low as possible and does not pose any health risks. In addition to legionella, which mainly enter the water through contaminated pipes or outdated installations, coliform germs and enterococci are the most common. These intestinal bacteria enter the water environment mainly from human or animal faeces. In most cases they are not hazardous to health but serve as an indicator of faecal drinking water contamination in water analyses.

Legionella in the water pipes
Legionella are among the most well-known germs in water. The moist germs multiply particularly well at temperatures between 20 and 25 degrees and are therefore mainly found in hot water systems. The biofilm in the pipes serves as an ideal breeding ground. Legionella enter the human body as an aerosol when water containing bacteria is inhaled and can trigger what is known as Legionnaires’ disease, a form of pneumonia. Especially in the summer months, house pipes and water pipe systems in holiday homes are often affected. For example, if the holiday home is empty for a long time, boilers or air conditioning systems are potential breeding grounds for legionella. Holiday home owners should therefore regularly turn on the taps and let the hot water run for a while – even in the shower, as legionella die off at temperatures above 60 degrees Celsius.

coli bacteria in drinking water
Escherichia coli bacteria, also known as E. coli, are among the coliform germs (coli bacteria) that are not allowed to occur according to the Drinking Water Ordinance. The limit value is zero. E. coli is a widespread intestinal germ and a natural component of the human and animal intestinal flora. Basically it is not dangerous. If E. coli bacteria are ingested through the drinking water, this can lead to diarrhoea and other gastrointestinal problems. Often there are no symptoms and the bacteria are excreted again. However, risk groups such as pregnant women or people with weak immune systems should observe the safety precautions when drinking E. coli-contaminated drinking water. In addition, the detection of E. coli is an indicator that other germs can also be found in drinking water.

Enterococci in drinking water
Enterococci are lactic acid bacteria that occur in the human and animal intestines and support the digestive processes there. The bacteria themselves cannot multiply in drinking water, only in the human or animal intestine. As with E. coli, the limit for enterococci in drinking water is zero. As an indicator organism, enterococci contamination in drinking water points to other potentially pathogenic bacteria. A quick water analysis and appropriate measures are therefore mandatory. Unlike coliform germs, enterococci can survive for a few weeks outside the intestine. In the case of contamination with these germs, it is difficult to define the time of contamination. If the bacteria are ingested through drinking water, they can lead to urinary tract infections or inflammation of the fallopian tubes in women. Wound infections and abdominal abscesses are also possible consequences of an enterococci infection.

How do the germs get into the drinking water?

Basically, the water quality in Switzerland meets very high standards. For example, the environmental organization WHO obliges Switzerland to report all drinking water data every three years. In addition, laws and standards regulate the limit values that must be observed in order to ensure water quality in the future.

The cause of germs in drinking water cannot always be traced back. While legionella in drinking water usually thrive on the biofilm in the water pipes themselves, bacteria such as E. coli  and enterococci enter the drinking water systems from outside. If they are found in water analyses, they are considered an indicator of faecal drinking water contamination.

Contamination in drinking water is often detected after very heavy rainfall. With normal rainfall, the layer of earth serves as a reliable filter. If there are large amounts of water at once, this mechanism does no longer work. Storms such as heavy rain can overload the sewage treatment plants or flood the sewage system. As a result, bacteria from the wastewater can get into rivers or bodies of water from which drinking water is obtained.

Other reasons for faecal drinking water contamination are accidents, renewal work on the main water connectors  or new connections in the house. Old sewers or leaking cesspools can also cause wastewater and drinking water to mix and lead to E. coli or enterococci contamination.

Contaminated drinking water – how is the quality restored?

Normally microbial contamination is quickly detected and the contaminated drinking water is not even fed into the network. In some cases, the water is also sterilized and cleaned as a prophylactic safety measure – for example during floods. If contaminated drinking water is suspected, only a professional drinking water analysis can provide information about the contamination with microorganisms. If germs are detected, the corresponding wells are provided with appropriate information boards until normalization. The water is discarded – it is directed into a stream instead of a well.

In the case of drinking water contamination due to leaks, it is important to locate the leak as a source of contamination, flush the pipes thoroughly and seal them. Multiple follow-up samples ensure that no more contamination can be found. In the event of serious incidents, a chlorine-containing network protection is added and, if necessary, a boil-off regulation is issued. The population in the affected areas is informed via the media that the water from the pipes can be contaminated and is only suitable for consumption after boiling. It can take several days before the drinking water has the usual quality again.

Drinking water quality: laws and obligations for water suppliers

The water quality in Switzerland is regulated nationally, the supply runs on a cantonal basis. The legal requirements for drinking water are regulated by a large number of laws and ordinances. The drinking water must meet the legal requirements defined in the Food and Commodities Ordinance (LGV) and in the EDI Ordinance on Drinking Water and Water in Publicly Accessible Baths and Shower Facilities (TBDV). The water suppliers and the owners of public buildings are responsible for the quality of the drinking water and must have this checked regularly, for example by independent special laboratories such as Biolytix.

The cantonal water suppliers are obliged to set up a quality assurance system to guarantee that the ordinances are observed. As part of the overall operational hazard analysis, they must periodically conduct an analysis of the hazards to water resources. The laboratories examine drinking water, well water, spring water and groundwater to guarantee microbiological safety in the water supply.